Phase II Test Summary

The full-scale test began on January 24, 2013. After the last crack appearance, the test continued for 538 cycles so that the transverse crack on the surface became incessant from the outer edges to the inner edges. The full-scale test concluded after a total of 4869 cycles, March 8, 2013.

Objectives

The primary research objective was to understand the mechanism of thermally-induced reflective cracking through full-scale tests. Test results were then compared to the field observations to determine if bottom-up reflection cracks propagated roughly 1 inch per year.

Overlay Design

Phase II test overlay consisted of two identical sections with a 1-ft gap. Both sections were constructed with two 2.5-in. lifts of standard FAA P-401 materials (PG 64-22).

Construction

To prevent AC-PCC interface slippage and secondary cracks, a thin tack coat of straight PG 64-22 asphalt was applied on the milled surface of two 12-in.-thick, 15- by 15-ft PCC slabs. A wood form was set prior to construction to divide the overlay into two 5.5-ft-wide strips with a 1-ft gap. The overlay was constructed with two 2.5-in. lifts of standard FAA P-401 materials (PG 64-22).

Instrumentation

Instrumentation took place in two phases: during construction and post-construction. During the overlay paving, H-type asphalt strain gages (EG) were installed at the bottom of each lift. These embedded gages were placed close to the outer and inner edges where the first reflection crack would most likely occur. Unfortunately, 5 of the 12 EGs malfunctioned after construction. Phase I test demonstrated the superior performance of surface strain gages (SG) and crack detectors (CD) for crack detection. The CD is simply a single strand of copper wire, and any erratic change in the output signal (i.e., voltage) indicates a discontinuity. However, due to the lack of durability, the best application for the SGs and CDs was on the pavement surface. Once the pavement temperature stabilized, 18 SGs and 18 CDs were installed at various locations. Note that all sensors were directly above and perpendicular to the PCC joint. Thermocouples were embedded at three depths (surface, mid-depth, and bottom) in the overlay to acquire the temperature profile.

Testing Protocol

Temperature variations were approximated by a haversine waveform describing the relationship between the joint opening and cycle time:

where t is the time of interest, D is the amplitude of joint opening, T is the cycle time, and R is rest period, which was included at the end of each loading cycle to allow the HMA materials to relax. In Phase II testing, the joint opening (D) was set at 12 mils which corresponded to a large temperature drop (17oF) at the overlay bottom in the field. Each haversine loading cycle began with a loading time of 75sec, once the actuators reached the maximum horizontal displacement (joint opening), a 75-sec unloading was executed and then followed by a rest period of 600sec to allow the overlay to relax.

Test Results

Crack Initiation

In Phase II test, three types of sensors were installed in the overlay to capture the crack propagation: surface strain gages, embedded strain gages, and crack detectors. At the gage location, the repeated loading-associated failure strains varied. As shown in Figure 1, the embedded strain gages at the overlay bottom recorded an average failure strain of 1900 microstrains. When a bottom-up crack reached the middle of the overlay, the failure strain ranged from 758 to 1075 microstrains. The failure strain on the pavement surface further reduced to approximately 550 microstrains. It can be concluded that, as part of the upward crack propagation process, a substantial amount of energy was dissipated.

Figure 1. Strain responses and failure strains.

Crack Propagation

When an instrumented sensor detected a crack forming under operating conditions, a silver marker was used to trace any crack progress on the pavement. In the horizontal plane, the resulting transverse crack was directly atop the underlying PCC joint (Figure 2a). Cracks at the outer and inner edges were vertical through the HMA overlay, as shown in Figure 2b. Apparently, the controlling fracture Mode for thermally-induced reflection cracks was fracture Mode I.

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Figure 2. Crack propagation.

The first reflection crack initiated at the overlay bottom of the south strip. One hundred cycles later, one of the bottom embedded strain gages detected a crack on the north strip. This delay was most likely caused by the uneven resistance from the HMA overlay. The first two cracks occurred at the same location, 6 in. from the outer edge. While the north strip required more cycles to propagate cracks, both strips exhibited a parallel pattern of the crack evolution. The crack length progressed aggressively once the crack reached the middle of the overlay. This observation simply implies that, given a specific pavement structure and materials, the critical zone to retard bottom-up cracks is the lower portion of the overlay.

Field experiences indicate that reflection cracks usually propagate to the pavement surface at a rate of approximately 1 in. per year. Assuming a 2-month (60-day) period of severe temperature variations, approximately 300 thermal cycles would be required for a reflection crack to travel through a 5-in.-thick HMA overlay. The corresponding crack propagation rate would be nearly 1 in. per 60 cycles. Figure 3 shows the crack evolution at two locations. The first through crack on the north strip was recorded by EG-4, EG-3, and CD-6. On the south strip, responses from EG-8, EG-7, and SG-14 indicated the second full-depth crack. Both cracks followed a linear growth and the propagation rates were 1 in. per 158 and 126 cycles for the north and south strip, respectively. Note that these rates from the full-scale test only imitate the effect of temperature loading. Therefore, if the pavement is subjected to both temperature and traffic loads, the “1 in. per year” may yield less conservative estimations.

Figure 3. Crack evolution.

Post-Test Forensic Study

Field cores were taken to verify the crack direction. For each overlay strip, three 9-in.- and one 6-in.-diameter cores were obtained. Figure 4a shows the locations of these cores. The 9-in.-diameter core ring was selected because it was easy to extract at the pavement edges. Each core was visually inspected to determine the crack extent and direction. As shown in Figure 4b, all cores exhibited a through bottom-up crack, except for the boundary between the cores close to the inner edges. On the boundary (red solid line), the cracks began from the surface and penetrated barely into the bottom 2.5-in.-thick HMA lift. To further investigate this observation, four 9-in.-diameter cores (N1, N2, S1, and S2) were sliced into 3-in. pieces. Because there were embedded strain gages located at 6 in. (EG-1, EG-2, EG-5, and EG-6) and 10 in. (EG-10 and EG-11) from the inner edges, the saw-cut went through these gages, as shown in Figure 4c. The saw-cut planes (blue dashed line) clearly show full-depth cracks. Consequently, it was speculated that the existence of strain gages interfered with the localized strain responses and somehow initiated a top-down crack in between, as hypothesized by the red arrows in Figure 4c.

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Figure 4. Forensic investigation.

Conclusions

Based upon the full-scale test results, the following conclusions can be drawn:

A two-strip overlay structure is suitable for evaluating reflective cracking control techniques; for example, the control section versus the mitigation section.

Phase II Test provided some insight towards the mitigation of thermally-induced reflective cracking. Instrumentation data revealed that once bottom-up reflection cracks reached a critical length, the crack evolution became very aggressive. For that reason, it is logical to sandwich a strain relieving HMA interlayer between the PCC slabs and the new overlay to minimize overlay stresses and to tolerate horizontal movements at the joint. Further exploration is needed to assess the impact of instrumentation on crack development.